The present invention relates to a spectrophotometer of a double beam system used for a general purpose.
A spectrophotometer measures a spectrum of light transmitted through a sample to be measured, and examines a wavelength of light absorbed by the sample or a wavelength of the transmitted light, to thereby analyze components of the sample.
Among the conventional spectrophotometers, there is a spectrophotometer with a double-beam system which carries out qualitative and quantitative analyses by measuring a ratio of an energy of light passing through a measured sample relative to an energy of light passing through a reference sample. In this case, light has to be irradiated to two optical paths. Normally, after light from a single light source is processed to have monochromatic light by a monochromator, the monochromatic light is divided into two optical paths, that is, signal light irradiated to the sample and reference light irradiated to a reference sample (double beam). Also, a detector which converts light energy into electric signals is used, but elements of the detector have to repeatedly measure an energy at a time of illuminating light and dark signal at a time of non-illumination of light so as to detect the energy conversion of the measured light. In order to repeatedly switch between the signal light and the reference light, and illumination and non-illumination of lights, a sector mirror (chopper) rotatable at a fixed rotational frequency is used. In order to eliminate an effect of a power supply frequency, in other words, an effect of a hum, the rotational frequency of the sector mirror is synchronized with the power supply frequency by utilizing a synchronous motor.
FIG. 2 shows a schematic structural view of a sector mirror. In a sector mirror 21, opening sections 22 and 23, mirrors 24 and 25, and shielding sections 26, 27, 28 and 29 are alternately arranged. The sector mirror 21 is disposed such that the opening sections 22 and 23 or the mirrors 24 and 25 are located in the optical path, and the sector mirror 21 is rotated at a fixed speed by a motor or the like. FIG. 3 shows a schematic structural view of a spectrophotometer of a double beam system. Light from a light source 31 is processed to provide monochromatic light by a monochromator 32, and then reaches the sector mirror 21. The sector mirror 21 is disposed at an angle of 45 degrees with respect to the optical path such that the opening sections 22 and 23 or the mirrors 24 and 25 are located in the optical path, and the sector mirror 21 is rotated at a fixed speed by a motor 34. By rotating the sector mirror 21, when the opening section 22 is located in the optical path, the light from the light source passes through the sector mirror 21, and after the light is transmitted through a sample 36, the light is reflected by a mirror 39 to be detected by a detector 42.
Next, when the sector mirror 21 is rotated, the shielding section 26 intercepts the light from the monochromator 32, so that the light does not reach the detector 42, resulting in that the detector 42 detects a dark signal. Thus, the sector mirror 21 also has a chopper function. By rotating the sector mirror 21 furthermore, the mirror 24 is disposed in the optical path, so that the light from the monochromator 32 is reflected by the mirror 24. Further, after the light is reflected by a mirror 38, the light is transmitted through a standard sample 37, and then the light is reflected by a mirror 40, to thereby be detected by the detector 42.
Also, by further rotating the sector mirror 21, the shielding section 27, the opening section 23, the shielding section 28, the mirror 25 and the shielding section 29 are located in the optical path in turn, so that the intensity or strength of the light transmitted through the sample 36, and the intensity or strength of the light transmitted through the standard sample 37 are repeatedly measured by the detector 42.
In the spectrophotometer, especially in a high-grade spectrophotometer, a spectroscopic analysis is carried out in a wide wavelength range which extends from an ultraviolet region to an infrared region, and if the wavelength range is widened, it is difficult to detect the entire wavelengths by a single detector. For example, in case the measurement is carried out in a range from a wavelength of 190 nm to a wavelength of 3200 nm, there are used two kinds of the detectors using PMT (photomultiplier) and PbS (lead sulfide). In general, these two kinds of detectors are switched over around the wavelength of 850 nm, so that the detector using PMT operates in the wavelength range from 190 nm to 850 nm, and the detector using PbS operates in the wavelength range from 850 nm to 3200 nm.
In case two kinds of the detectors using PMT and PbS, are used, their responses are extremely different (for order of several digits). Especially, in case the detector using PbS is cooled to increase the detection sensitivity, a response speed thereof becomes extremely slow. As the speed of rotating the sector mirror becomes faster, time required for scanning the wavelength in a necessary range can be shorter, so that a short time analysis is available. However, in the detector which is slow in the response, it is impossible to output a sufficient signal by increasing the speed of the rotation of the sector mirror. Therefore, in the conventional spectrophotometer, the rotational frequency of the sector mirror is set at the speed which does not cause the problem in the response speed of the detector, and accordingly, a wavelength scanning speed is determined.
Since the rotational frequency of the sector mirror is fixed at the constant value, in case the two kinds of the detectors are used, the rotational frequency of the sector mirror is determined based on the detector which is slow in the response speed, and it is inevitable for the analysis time to become long. In the spectrophotometer which measures the wide wavelength range from the ultraviolet region to the infrared region, 1800 rpm, which is synchronous with the power supply frequency, is generally adopted as the rotational frequency of the sector mirror.
The present invention has been made to solve the aforementioned problems, and an object of the invention is to provide a spectrophotometer, which includes two kinds of detectors in order to analyze a wide wavelength range and outputs sufficiently large signals while the analysis time can be shortened.
Further objects and advantages of the invention will be apparent from the following description of the invention.
To achieve the aforementioned object, the present invention provides a spectrophotometer, in which a rotational frequency of a motor for rotating a sector mirror can be controlled externally. In the spectrophotometer of the invention, when a detector which is fast in a response speed is operated, the rotational frequency of the sector mirror is increased, to thereby shorten time for scanning the wavelength. When a detector which is slow in a response speed is operated, the rotational frequency of the sector mirror is decreased, so as to obtain a sufficiently large output signal. As described above, by controlling the rotational frequency of the sector mirror in accordance with the response speed of the detector, there can be obtained the spectrophotometer in which the analysis time is short and the output signal is sufficiently large.